For liquid crystal display devices, the classification based on the operating mode of liquid crystal molecules includes modes such as phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), fringe field switching (FFS), polymer sustained alignment (PSA) and field induced photo-reactive alignment (FPA). The classification based on the driving mode of the device includes passive matrix (PM) type and active matrix (AM) type. The PM type is classified into static type, multiplex type and so forth, and the AM type is classified into thin film transistor (TFT) type, metal-insulator-metal (MIM) type and so forth. The TFT type is further classified into amorphous silicon type and polycrystal silicon type. The latter is classified into high temperature type and low temperature type according to the production process. The classification based on the light source includes the reflection type utilizing natural light, the transmission type utilizing a backlight, and the semi-transmission type utilizing both natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. This liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to give an AM device having good general characteristics. Table 1 below summarizes the relationship between these two groups of general characteristics. The general characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. The desirable maximum temperature of the nematic phase is about 70° C. or higher, and the desirable minimum temperature of the nematic phase is about −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Composition and AM DeviceGeneral Characteristics ofGeneral Characteristics ofNo.CompositionAM Device1wide temperature range ofwide temperature range in whichnematic phasedevice can be used2small viscosity 1)short response time3suitable optical anisotropylarge contrast ratio4large positive or negativelow threshold voltage and smalldielectric anisotrophyelectric power consumptionlarge contrast ratio5large specific resistancelarge voltage holding ratio andlarge contrast ratio6high stability to ultravioletlong service lifelight and heat7large elastic constantlarge contrast ratio and shortresponse time1) A composition can be injected into a liquid crystal display device in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. The suitable value of the product depends on the kind of operating mode. The suitable value is about 0.45 μm in a device having a mode such as TN. In a device having a VA mode, the suitable value is in the range of about 0.30 μm to about 0.40 μm. In a device having an IPS mode or an FFS mode, the suitable value is in the range of about 0.20 μm to about 0.30 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large positive or negative dielectric anisotropy of the composition contributes to a low threshold voltage, small electric power consumption and a large contrast ratio of the device. Accordingly, a large positive or negative dielectric anisotropy is desirable.
A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, it is desirable that a composition should have a large specific resistance at room temperature and also at a temperature close to the maximum temperature of nematic phase in the initial stage. It is desirable that a composition should have a large specific resistance at room temperature also at a temperature close to the maximum temperature of nematic phase after being used for a long time. The stability of the composition to UV light and heat relates to the service life of the device. When the stability is high, the device has a long service life. A large elastic constant of the composition contributes to a large contrast ratio and a short response time. Accordingly, a large elastic constant is desirable. Such characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so on.
A composition having positive dielectric anisotropy is used for an AM device having a TN mode. In contrast, a composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode or an FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device having a PSA mode or an FPA mode.
A composition tends to deteriorate due to oxidation when the liquid crystal display device is used for a long time. Hence, an antioxidant such as 2,6-di-tert-butyl-4-methylphenol (BHT) is sometimes added to prevent oxidation of the composition (Patent Documents No. 1 and No. 2). A more useful antioxidant is expected in view of the solubility in a composition for instance.